1. Field of the Invention
This invention relates to moisture-curable silicone compositions, and methods for preparing such compositions.
2. Description of the Related Art
Moisture-curable silicone sealants in the form of thixotropic pastes are known. The user generally applies the paste by extruding it from a tube, aerosol can, caulking cartridge, or automatic pumping equipment onto a surface to be sealed and/or mated. One problem with such pastes is that they are messy and difficult to apply reliably and accurately. In addition, once applied the paste can be readily dislodged. The paste must also be allowed to xe2x80x9cskin overxe2x80x9d or xe2x80x9cset upxe2x80x9d once applied, necessitating a waiting period between the time the sealant is applied and the time the object to which the paste is applied can be used.
Solid sealant strips made of Gore-Tex(copyright) microporous polytetrafluoroethylene are also known. Such strips create a seal by conforming to surface imperfections and filling gaps between mating surfaces, rather than adhering to the surface. The inability of such strips to adhere to the surface, coupled with a relatively low compression set resistance, creates a tendency for the seal to fail when the mated surfaces move relative to each other. In addition, these strips do not have sufficient adhesive strength to seal over a hole or gap in a surface.
In a first aspect, the invention features a moisture-curable silicone sealing composition having a storage shear modulus of at least 9.0 KPa and a loss shear modulus of at least 4.0 KPa prior to curing when measured at 25xc2x0xc2x11xc2x0 C. at a frequency of 0.1 rad/sec. Unlike thixotropic pastes and solid sealant strips, the composition is tacky and self-supporting at room temperature prior to curing. Preferably, the composition further exhibits a storage shear modulus of at least 80 KPa and a loss shear modulus of at least 50 KPa prior to curing when measured at 25xc2x0xc2x11xc2x0 C. at a frequency of 100 rad/sec.
In a second aspect, the invention features a moisture-curable silicone sealing composition having a viscosity of at least 98.5 KPa Sec prior to curing when measured at 25xc2x0xc2x11xc2x0 C. at a frequency of 0.1 rad/sec. Unlike thixotropic pastes and solid sealant strips, the composition is tacky and self-supporting at room temperature prior to curing. Preferably, the composition further exhibits a viscosity of at least 940 Pa Sec prior to curing when measured at 25xc2x0xc2x11xc2x0 C. at a frequency of 100 rad/sec prior to curing.
In preferred embodiments of both aspects, the sealing composition is in the form of a ribbon, rope, or sheet. The sealing composition preferably has an elongation value of about 200 to about 1200% (preferably about 350 to about 600%) after curing.
Preferred compositions include a hydroxy-terminated polydialkylsiloxane, fumed silica, and a silane crosslinking agent. The polydialkylsiloxane preferably is present in an amount ranging from about 35 to about 85% by wt. (more preferably from about 65 to about 75% by wt.). The fumed silica preferably is present in an amount ranging from about 5 to about 30% by wt. (more preferably from about 20 to about 25% by wt.), and has a surface area of at least 50 m2/g (preferably ranging from about 130 m2/g to about 400 m2/g). The silane is preferably present in an amount ranging from about 1 to about 15% by wt. (more preferably from about 2.5 to about 6% by wt.). The sealing composition is preferably essentially free of curing catalysts.
The invention further features methods of sealing two surfaces together by applying the above-described sealing compositions to the first surface, and then contacting the first surface with the second surface in the presence of moisture to bond the two surfaces together.
Throughout this application the following definitions apply:
A xe2x80x9ctackyxe2x80x9d composition is a composition which passes the Tack Test described infra under Test Procedure D.
A xe2x80x9cself-supportingxe2x80x9d composition is a composition which passes the Stretch Test described infra under Test Procedure C.
The invention provides tacky, self-supporting sealing compositions which (unlike thixotropic pastes) can be readily and accurately applied by hand because they do not transfer to any substantial extent to the user""s hands. Because the sealing compositions are self-supporting, they can be provided, e.g., in the form of ropes, ribbons, or sheets stored in laminated packages and conveniently dispensed simply by lifting the rope, ribbon, or sheet out of the package. The compositions can then be formed by hand into a variety of shapes to seal a number of mating surfaces. In addition, the sealing compositions are functional upon application; there is no substantial set-up time following application. The invention thus combines the ease of handling associated with solid sealant strips with the sealing behavior of curable thixotropic pastes.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.
Preferred silicone sealant compositions according to the invention contain (a) 35-85% by wt. of a hydroxy-terminated polydialkylsiloxane; (b) 5-30% by wt. fumed silica; and (c) 1-15% by wt. of a silane crosslinking agent. The particular amounts of ingredients, and the conditions under which they are processed, are selected such that the uncured composition is tacky and self-supporting (and thus readily handled, particularly when provided in the form of a ribbon, rope, or sheet). In general, it is desired that the composition be tacky enough to adhere to a clean metal substrate, yet not be so tacky that it transfers to the user""s hands. Similarly, the composition must have sufficient integrity to retain its shape, but must not be so stiff that it cannot be easily formed by hand into the shape needed for a-particular application.
The viscosity and storage and loss shear moduli of the sealing composition measured at low (0.1 rad/sec) frequency according to Test Procedure A provide a quantitative measure of the xe2x80x9chandlabilityxe2x80x9d of the sealing composition. It is generally preferred that the composition exhibit a viscosity of at least 98.5 KPa Sec at 0.1 rad/sec, and a storage shear modulus of at least 9.0 KPa and a loss shear modulus of at least 4.0 KPa at 0.1 rad/sec.
The viscosity and complex shear modulus of the sealing composition measured at high (100 rad/sec) frequency according to Test Procedure A provide a quantitative measure of the xe2x80x9cprocessabilityxe2x80x9d of the sealing composition. It is generally preferred that the composition exhibit a viscosity of at least 940 Pa Sec at 100 rad/sec, and a storage shear modulus of at least 80 KPa and a loss shear modulus of at least 50 KPa at 100 rad/sec.
Any of the hydroxy-terminated polydialkylsiloxanes typically used in known sealing compositions may be used in the sealing compositions according to the invention. One class of preferred materials includes polydialkylsiloxanes having viscosities at 25xc2x0 C. ranging from about 0.08 m2/s to about 0.3 m2/s. Examples of commercially available polydialkylsiloxanes failing within this class include those available from PPG Industries, Inc. under the designations Masil SFR 80,000, SFR 150,000, and SFR 300,000, and those available from Huls America of Piscataway, N.J. under the designation PS348.7. Hydroxy-terminated polydialkylsiloxanes having lower viscosities may also be used if the viscosity of the polymer is increased in situ during processing.
Fumed silicas having relatively high surface areas (e.g., in the range 50-400 m2/g) are generally preferred. Such silicas are commercially available from Cabot Corp. of Tuscola, Ill. under the trade designations L-90, LM-130, LM-150, MD-7, MS-55, MS-75D, H5, M5, HS-5, EH-5, TS720, TS530, and TS610, and from Degussa Corp. of Dublin, Ohio under the trade designations Aerosil 90, 130, 150, 200, 300, 380, R202, R810, R811, R812, R975, R976, R972, and R974.
Particularly preferred are hydrophobic fumed silicas (i.e., fumed silicas provided with a hydrophobic coating). Such silicas have a reduced tendency to adsorb moisture, and thus introduce less moisture into the composition, compared to silicas lacking a hydrophobic treatment; exposure to moisture is undesirable because it initiates premature cure. Also preferred are densified fumed silicas. Such silicas facilitate compounding and processing because they are relatively easy to handle. Examples of densified fumed silicas (both of which are commercially available from Degussa Corp.) include R974V (having a bulk density of 120 g/l and a surface area of 170 m2/g) and R972V (a hydrophobic fumed silica having a bulk density o 120 g/l and a surface area of 110 m2/g).
Suitable silane crosslinking agents generally have the formula RnSiY4xe2x88x92n where R is a monovalent hydrocarbon group (e.g., an alkyl, alkenyl, aryl, or alkaryl group);, n is 0, 1, or 2; and Y is a monovalent hetero-alkyl or aryl group such as a dialkylketoxamino group (e.g., methylethylketoxamino, dimethylketoxamino, or diethylketoxamino), alkoxy group (e.g., methoxy, ethoxy, or butoxy), alkenoxy group (e.g., isopropenoxy), acyl group (e.g., acetoxy), alkamido group (e.g., methylacetamido or ethylacetamido), or arylamido group (e.g., benzylamido). Silane crosslinldng agents falling within this category are commercially available, e.g., from Silar Laboratories of Scotia, N.Y. Particularly preferred silane crosslinking agents are dialkylketoaminosilanes because they exhibit good shelf-stability and do not form deleterious by-products upon cure. Examples include methyltris(methylethylketoxime) silane (xe2x80x9cMOSxe2x80x9d) and vinyltris(methylethylketoxime) silane (xe2x80x9cVOSxe2x80x9d), both of which are commercially available from Allied-Signal, Inc. of Morristown, N.J., and alkoxysilanes available from OSI Chemicals of Lisle, Ill.
The sealing composition may also contain adhesion promoters, plasticizers, curing catalysts, and/or fillers. The amounts of these ingredients are selected such that the tacky, self-supporting nature of the sealing composition prior to cure is maintained.
Examples of suitable adhesion promoters include alkoxysilanes such as methyltrimethoxysilane, vinyltrimethoxysilane, gamma-glycidoxypropylmethyldiethoxysilane, and gamma-mercaptopropyltrimethoxysilane. Suitable commercially available adhesion promoters include alkoxysilanes commercially available from OSI Chemicals of Lisle, Ill. under the designations A1120, A1170, Y11343, Y11542, and Y11597.
Suitable plasticizers include polydialkyl siloxane oils which are not hydroxy-terminated and therefore do not react with the other ingredients in the sealing composition during cure. Such plasticizers typically have viscosities in the range of about 0.0007 to 0.06 m2/s. Examples include siloxane oils commercially available from Huls America and the SF(trademark) series of silicone oils available from PPG Industries, Inc.
Examples of suitable curing catalysts include alkyl tin derivatives (e.g., dibutyltindilaurate, dibutyltindiacetate, and dibutyltindioctooate commercially available as xe2x80x9cT-series Catalystsxe2x80x9d from Air Products and Chemicals, Inc. of Allentown, Pa.), and alkyl titanates (e.g., tetraisobutylorthotitanate, titanium acetylacetonate, and acetoacetic ester titanate commercially available from DuPont under the designation xe2x80x9cTYZORxe2x80x9d). In general, however, it is preferred to select silane crosslinking agents that do not require the use of curing catalysts to avoid reducing shelf-life and adversely affecting the physical properties of the sealing composition.
Examples of suitable fillers include precipitated silica, silica aerogel, ground quartz, calcium carbonate, magnesium carbonate, kaolin and bentonite clays, talc, titanium dioxide, aluminum silicate, diatomaceous earth, ferric oxide, carbon black, zinc oxide, ceramic microspheres (e.g., microspheres commercially available from 3M Co. of St. Paul, Minn. under the designations Z Light Spheres(trademark), Zeeosphere(trademark), and Macrolite(trademark) Ceramic Spheres), glass microbubbles, and fibrous fillers (e.g., glass and ceramic fibres).
Other ingredients which may be added to the sealing composition include dyes, fungicides, heat stabilizers, fire retardants, ultraviolet stabilizers, modifiers such as MQ siloxane resins commercially available from General Electric Co. and MDQ siloxane resins commercially available from Shin-Etsu of Osaka, Japan, and thixotropes (e.g., thixotropes commercially available from Rheox, Inc. of Hightstown, N.J. under the designation xe2x80x9cThiatrol(trademark)xe2x80x9d and xe2x80x9cThixcin(trademark)xe2x80x9d). Again, the amounts of these additional ingredients are selected such that the tacky, self-supporting nature of the sealing composition prior to cure is maintained.
The sealing composition is prepared by mixing the components together in a high shear mixing process using equipment having closed mixing chambers to prevent exposure to moisture (and thus premature cure). The goal of the mixing process is wet the fumed silica particles as uniformly as possible with the polydialkylsiloxane, and to uniformly disperse the other ingredients. The high shear rates used during mixing achieve this objective by breaking up silica agglomerates and aggregates, and dispersing the particles substantially uniformly throughout the mix.
The high shear mixing process may be carried in a variety of ways, including by means of a press-mixer (e.g., a Model 10L NK PRESSMXER available from J. M. Voith AG of St. Polten, Austria) or a co-rotating twin extruder (e.g., a TYPE MC 5-co-rotating, intermeshing twin screw extruder available from Welding Engineers, Inc. of Blue Bell, Pa. or a co-rotating, intermeshing twin screw extruder available from Teledyne Readco of York, Pa.). In the case of compositions prepared in the press-mixer, the order in which the various components are added is important. Specifically, it is preferred to add a substantial portion of the fumed silica to the polydialkylsiloxane before adding the remaining ingredients.